wpd: simplify by using a generic engine worker pool

HISTORY.rst

cmd/rednerd/cmd/engine.go

cmd/rednerd/cmd/serve.go

engines/wpd/engine.go

engines/wpd/job_runner.go

engines/wpd/job_runner_test.go

engines/wpd/pool.go

rendering/pooled_engine.go

rendering/registry.go

tests/wpd_load_test.go